It is well known that the homogeneous metallocene catalyst comprising a metallocene compound and an activation agent is the most important new generation polyolefin catalyst following the Ziegler-Natta catalyst. Compares to the traditional Ziegler-Natta catalyst system, this kind of catalyst possesses many outstanding performances such as ultrahigh catalytic activity, narrow molecular weight distribution of the polymer, uniform composition distribution of the copolymer, and the possibility to control the stereochemical microstructure, etc. Although the homogeneous metallocene catalyst has so many outstanding performances, two problems have been come across in the industrial application that need to be urgently solved: one is the solubleness of the catalyst in the preparation medium and the liquid medium for polymerization reaction, the severe phenomenon to adhere the autoclave in the polymerization process, and the difficulty to control the particle morphology, resulting in the difficulty to be used in gas phase or slurry polymerization; the other is the need to use great amounts of promoters (mainly methyl aluminoxane, abbreviated as MAO) to achieve higher catalytic activity, causing a high production cost of the catalyst.
Presently, the major approach to reduce the production cost of the metallocene catalyst is activating the molecule of the catalyst with a substance having strong Lewis acidity to reduce the amount of promoters. However, this approach cannot improve the morphology of the polymer.
In order that the metallocene catalyst can be used in the gas phase polymerization process, the prior an uses the method of supporting the metallocene catalyst on a support to improve the particle morphology of the polymer. The support once used includes inorganic oxides the surface of which contains hydroxyl such as silica, alumina, etc, or porous polymer supports such as polyethylene, polypropylene and polystyrene. However, the costs of these supports are all rather high, and the particle morphology of the prepared polymer is not desirable, so an attempt has been made to use magnesium halide as the support for supporting metallocene compounds. For example, CN 1054776A discloses the use of magnesium chloride as the support of metallocene compounds, which is obtained by the reaction between tert-butyl chlorine and dibutyl magnesium in an aliphatic hydrocarbon solvent, and an electron donating compound such as isopentyl ether needs to be added, followed by impregnation of the support in an aromatic solvent containing metallocene compounds to yield the supported catalyst. A promoter, MAO solution in toluene, needs to be added when polymerization is carried out and the amount is Al/Zr=2500.
EP 878,483 and EP 879,484 disclose using the complex compound of magnesium chloride prepared by the decomposition of the Grignard reagent with tetrahydrofuran as the support and subsequently impregnating the support in a tetrahydrofuran solution dissolving a metallocene compound to yield the supported catalyst. A MAO solution in toluene also needs to be added when polymerization is carried out and the amount is Al/Zr=5000.
CN 1130914A discloses using spherical alcoholate of magnesium chloride as the raw material of the support; which is first physically dealcoholized with hot nitrogen till an alcohol content of 10–45%, and then the remaining alcohol is removed with an excessive alkyl aluminum compound (such as triethyl aluminum tri-isobutyl aluminum, trimethyl aluminum) to yield the support which is impregnated in the solution of the complex formed from a metallocene compound and alkyl aluminum or MAO to yield the supported catalyst. A great amount of MAO also needs to be added when polymerization is carried Out, and the amount is Al/Zr=4000.
As stated above, the prior aft typically uses impregnation method to directly support the main catalyst component, metallocene compound, on the magnesium halide support or treats the magnesium compound support first, and then supports the metallocene compound or its complex with alkyl aluminum or MAO on the treated support. When used in the olefin polymerization, a great amount of soluble MAO is added. Although a highly active catalyst can be obtained the use of a great amount of MAO not only results in an increase in the cost, but also dissolves the metallocene compound having been supported, on the support making a part of metallocene compound separate from the support dissolve in the polymerization medium, and form a homogeneous catalyst, thereby leading to a poor particle morphology of the polymer.
Therefore, if the process proceeds inversely, i.e. MAO is first combined with magnesium halide support and then the m tallocene compound is supported on the magnesium halide support, not only the above shortcoming of the dissolving and separating of the metallocene compound from the support due to the addition of a great amount of MAO can be overcome, but also only a small amount of MAO is needed to fully activate the metallocene compound on the support, thereby reducing the cost of the catalyst and enhancing its activity. But because there exists hydroxyl on the surface of the support such as silica, etc, MAO many connect with the support with chemical bonds through reaction with hydroxyl, and therefore it hard to separate from the support when polymerization is carried out. As for a metallocene catalyst with magnesium halide as the support because there is no hydroxyl on the surface of the support, it cannot connect with MAO by chemical bonds as silica, and therefore the supporting of MAO on magnesium halide is not firm. Therefore, from the teaching of the prior art, the method that MAO is first supported, and then metallocene is supported is not suitable for magnesium halide. Therefore, nobody makes such an attempt in the prior art, in other words, the prior art has not disclosed the combination of magnesium halide with MAO to form corresponding catalyst component.
However, the inventors of the present invention have unexpectedly discovered that sufficient aluminoxane can be firmly supported on magnesium halide by adding a multifunctional organic compound.